1. Field of the Invention
This invention relates generally to focusing apparatus and is concerned more particularly with apparatus for automatically maintaining a laser beam focused on a surface of a workpiece.
2. Discussion of the Prior Art
A relatively high energy laser beam may be employed advantageously for precision removal of material from selected areas of a workpiece. In the manufacture of semiconductor devices, for example, a high intensity laser beam may be used for trimming microcircuit resistive elements or for vaporizing conductive links deposited on a dielectric substrate to produce a microcircuit having a desired configuration. The laser beam also may be used for burning holes of accurate size in covering layers of dielectric material to expose specified areas of conductors and accomplish a desired task, such as trimming or vaporizing, for example.
Consequently, a semiconductor device may be suitably mounted on a computer controlled work table for moving along X and Y coordinates in a plane which extends transversely through the path of the laser beam. While traversing the path of the beam, selected surface areas of the device may be positioned in alignment with the laser beam for respective predetermined intervals of time sufficient for the beam to accomplish the desired task in each of the selected areas. However, it may be found that for numerous reasons, such as irregularities in the surface of the workpiece or in the movement of the table, for examples, some of the selected surface areas of the workpiece are out of focus with the laser beam. As a result, holes drilled in overlying layers of dielectric material may be oversize and extend over into undesired surface areas of the device. Also, underlying microcircuit elements may not be adequately trimmed or conductive links may not be completely vaporized to produce the circuit having the desired configuration.
Thus, material removal systems of the described type may advantageously be provided with means for automatically maintaining the laser beam in-focus on the selected surface of semiconductor device. The prior art shows a number of assemblies designed for maintaining a beam in-focus on a target surface. Some of these prior art assemblies are dependent on variations in a return angle of a beam reflected from the target surface, which may prove to be unsatisfactory when the target surface is specular. Other prior art assemblies are based on producing minute variations in the focal distance to determine resulting differences in image intensity or contrast from which is derived an error signal indicative of an out-of-focus condition. However, the resulting differences in image intensity or contrast may be partly due differences in target surface reflectivity; and the apparatus required for producing the modulations in focal distance may be quite complex.
Therefore, it is desirable to provide a focusing apparatus having automatic means which is virtually insensitive to amplitude variations caused by differences in reflectivity of the target surface or to return angle deviations of a reflected beam, and does not require minute modulations of the focal distance to determine corrective motion required for maintaining a beam in-focus on a target surface.